Thursday, February 3, 2011

Basic Computer Networking Concepts

Modulation is the technique by which data is turned into an analog wave or a digital signal. There are different techniques of doing this. Amplitude modulation mixes a plain carrier wave with another data wave to change the amplitude of the carrier wave. Frequency modulation changes the frequency of the carrier wave to encode digital data into the carrier wave. A fast frequency may stand for a one whereas a normal frequency means zero. The most complicated type of modulation is analog modulation that uses phase to encode data. This changes the wave's starting point to encode data.

Analog Modulation is used in AM radio. Frequency Modulation is used in FM radio. Old broadcast television uses analog modulation for video, frequency modulation for sound, and phase modulation for color.
Digital modulation uses much less power than analog modulation, and is one of the reasons why small battery powered devices tend to use digital modulation. In digital modulation a clock is continuously sending on pulses, such that the signal looks like it is turning on and off extremely fast. The on and off signal that needs to be transmitted is mixed into these clock pulses.
Multiplexing is the technique by which multiple network communications are transmitted on a single physical wire. It can be compared to a highway with multiple lanes and cars. In the physical electronic version multiple signals are combined into a single signal using a multiplexer. On the other end of the wire a demultiplexer performs the reverse operation to break the single signal into multiple signals again.
Time division multiplexing gives each signal using the wire an equal share of time to communicate over the wire. After one signal has used its share of time the next signal starts using the wire. These single shares of time are extremely short so that it appears as if all signals are using the same wire at the same time. Time division multiplexing is wasteful, however, because if one signal does not have anything to communicate it still gets a share of the wire time, which is then wasted.

Statistical multiplexing is more complicated. It measures how much each signal needs to use the wire, and signals with more data to transmit get a greater share of time using the wire. This maximizes the bandwidth by ensuring that very little time is given to signals that have very little data to transmit.
Frequency division multiplexing gives each signal is own frequency to communicate it. This allows multiple signals to communicate at the same time, rather than having to divide wire usage time into dedicated segments.
Wavelength division multiplexing is used for fiber optic multiplexing. Beams of light from different carrier waves are mixed into a single beam of light, as if by a prism. On the other end the wavelength demultiplexer works like a prism to split the light into different wavelengths again. Dense wavelength division multiplexing is an extreme version of wavelength division multiplexing, which breaks light up into 80 to 160 separate channels. It requires very high quality equipment that is extremely expensive.
Simplex, Half-Duplex, and Full-Duplex
Simplex network communication is only one direction. An example may be television, radio, or a baby monitor. You can receive communication but you can not send communication. In half-duplex network communication data can travel in both directions, but only in one direction at a time. An example may be a walkie-talkie in which you can send and receive, but not do both at the same time. In full-duplex communication you can send and receive at the same time, for example on a phone.
Point to Point versus Point to Multipoint
In point to point communication one sender is communication with one receiver. In point to multipoint communication one sender's signal is heard by many receivers. This is also called broadcast or multicast communication as opposed to non-boradcast point to point communication.
Throughput versus Bandwidth
These two terms are often used interchangeably, but technically they mean two different things. Bandwidth refers to the range of frequencies used on a channel. This range is closely correlated with the possible speed of the communication. Throughput is the actual measure of how many bits you can transmit per second. Because of the close relationship between bandwidth and throughput the term bandwidth is often used to mean throughput.
Throughput is reduced by transmission flaws such as noise, which may result in transmission errors that have to be corrected; and latency, which slows communication times.
Baseband versus Broadband
Baseband is a transmission form in which there is only one communication channel. Broadband has multiple channels. An example of this might be cable television.
Transmission Flaws
Noise is caused in a communication by electromagnetic interface, cross talk between wires, radio frequency interface. Noise causes analog waves to no longer be smoothly curved.

In a digital communication noise causes the square on and off state of the network to appear slightly jagged. A analog signal is extremely hard to clean of noise. Amplifiers to boost the analog signal may even exaggerate the noise. With a digital signal repeaters can effectively clean noise from the signal completely. Noise can also be limited by shielding wires with metal conduit, or choosing network mediums which are more resistant to noise.
Attenuation is the loss of a signal's strength. The further the signal gets from the source the less distinct it becomes. Amplifiers for analog signals and repeaters for digital signals boost signals to remove attenuation. Amplifiers and repeaters are simple devices that fit in the physical layer of the OSI model. They have not understanding of what they are amplifying or repeating.
Latency is a third transmission flaw. The farther away from each other two network hosts are the more latency there will be. Latency depends on the type of network used. Fiber optic will have less latency than copper wire networks, which will in turn have less latency than a wireless network. Latency can also be caused by bottlenecks and slow hardware on a network.